The Make-Believe Mystery — Carolyn Keene

It's hard to write a mystery story when the plot disappears!
Nancy, Bess, George, and Phoebe Archer team up to write a mystery for a class project. Everyone else in the class is on teams too, and the best story will be printed in the school newspaper. First, snooty Brenda Carlton tells the girls they have no chance of winning -- then the notebook in which they wrote their mystery disappears!
The girls had a neat idea for a ghost story, and now another team may have stolen it. Things get weird when they find notes written in drippy red writing -- with words that come straight from their story! Is the make-believe mystery coming true?